In order to purify a methane-rich stream coming from an organic source, so as to produce a purified product, it is necessary to remove the impurities, such a carbon dioxide, oxygen and nitrogen. Ideally the product contains less than 2% carbon dioxide and less than 2% for the total oxygen and nitrogen content.
In this context, a methane-rich stream contains at least 30% methane.
All the composition percentages in this document are molar percentages.
Biogas, coming for example from an installation storing non-dangerous waste, is a mixture of methane, carbon dioxide, nitrogen, oxygen and traces of other impurities such as water and hydrogen sulfide or volatile organic compounds (VOCs).
For reprocessing methane as a biofuel or for injection into the natural gas system, purification is necessary. The impurities present in traces may easily be stopped in adsorption beds or other methods known to persons skilled in the art.
A few remarks concerning the presence of oxygen in natural gas are found in US-A-2006/0043000. The percentage of oxygen in natural gas does not exceed 0.1% according to other sources.
CO2 and CH4 are preferably separated by permeation in a membrane system. Membranes do not however make it possible to separate methane from the gases in air economically; however, it is necessary to comply with demanding purity requirements for injecting biogas into the natural gas system. It is then necessary to find a supplementary means for separating methane from gases in the air. Offers using an adsorption system for this are found at the present time on the market. This solution has several drawbacks, such as low efficiency, many wearing parts or very bulky adsorbent bottles and buffer vessels.
Another solution for separation is cryogenic distillation as described in WO-A-09/004207. This may achieve very high efficiencies, works continuously and requires only very little maintenance.
However, with the presence of oxygen in the mixture to be separated, the problem of flammability of the methane/oxygen binary is posed following the superconcentration of oxygen in the middle of the distillation column. Even very small quantities of oxygen in a feed far from being flammable accumulate in the column and may create a dangerous situation.
This problem has not been dealt with in the prior art, as can be seen from U.S. Pat. No. 2,519,955, where a gas containing oxygen (air) is actually deliberately introduced into a natural gas distillation column devoid of oxygen.
A catalytic deoxygenizer could solve this problem but gives rise to other problems such as the addition of a supplementary element in the method, the creation of water and CnHm or even carbon or a potentially lower reliability of the biogas purification assembly.